Optical imaging camera technology is currently dominated by 2-D Photo-Detector Array (PDA) sensor chips based optical imagers. These include Charge Coupled Device (CCD), and CMOS technology multi-pixel optical sensor chips and Focal Plane Arrays (FPAs). Such imaging units form excellent imagers under standard light conditions, particularly for visible band imaging applications, where silicon sensors produce excellent image quality. For bright light imaging applications, optical attenuators can be placed before the camera sensor unit. However, such an approach can alter and sometimes deteriorate the original image quality. In particular, it has been found that coherent laser radiation easily saturates CCD/CMOS image sensors, making them highly non-attractive for accurate laser beam irradiance mapping. Furthermore, bright zones within a normal lighting level scene can distort the scene captured by a CCD/CMOS chip camera, and consequently the scene can often be rendered unobservable.
To enable robust optical irradiance mapping, such as under high brightness conditions, a Digital Micromirror Device (DMD) can be used an agile pixel optical irradiance sampler. The DMD can direct sampled light in the optical irradiance 2-D map to a single point detector (PD) or a pair of single PDs. Such an imager design allows software programming of the size, shape, location, and temporal duration of the agile pixel that pixel-by-pixel sample the optical irradiance map present on the DMD plane. Thus, the function of a multi-pixel PDA chip with for example a million photo-detector pixels working in parallel is replaced by a multi-pixel shutter or mirror array device with a million micro-mirrors, and used in conjunction with a single point PD pair with the DMD operating in serial mode, one pixel at a time. Hence, inter-pixel crosstalk within a CCD/CMOS type device is essentially eliminated. This includes the elimination of pixel saturation and flooding due to high light levels at a given pixel.
It will be appreciated that the speed of image capture using a DMD depends on how fast the agile pixel can be scanned on the DMD plane to generate the adequate optical irradiance samples to electronically stitch together the collected image point sample data. Hence, the smaller the agile pixel, and the greater the number of samples, the longer it takes to generate an optical irradiance map. This is assuming that the reset time of the micromirrors on the DMD creates the time constraint versus the response time of the point PD and the data acquisition and image processing electronics. As a DMD is a broadband device (400 nm-2500 nm), a DMD imager also forms a versatile broadband imager, particularly for bright light conditions where attenuation of incident irradiance is to be avoided.
It is known to provide an active 3-D imager based on a laser beam with an Electronically Controlled Variable Focal length Lens (ECVFL) and a CCD/CMOS imager. It is also known to incorporate a dual PD DMD-based imager within such an active 3-D imager design using a CCD, laser, and ECVFL. In this imaging system, when the laser is on, the full DMD is set to its +θ state, so that on reflection from a target, light returns the same way to the CCD so the laser spot can be observed on the target. When the laser is turned off, the DMD can operate with both +θ state and −θ state at the same time, to enable the system to act as a point sampler to generate an image using the two point PDs under non-active illumination light conditions. In other words, this system acts either as an active imager using the laser and CCD with the DMD in one mirror state or as a passive imager using both the states of operation of the DMD along with the dual point PDs with the laser turned off.
Another known DMD-based imager places a single PD and a single PDA in each arm of two deflected light beam ports of a DMD, in order to realize a compressive imager where the PDA is a medium or small pixel count PDA (compared to the DMD), and is needed to view only a sub-set of the scene pixels in a larger pixel count scene normally viewed by one large pixel count PDA. This configuration can also be used with the two PDAs operating at different wavelengths compared to the point PDs, thus enabling a three wavelength band imager to be provided. In this configuration, the wavelength λ2 band image viewing uses the DMD operating in both its states with the two point PDs to implement serial sequence agile pixel scans across the DMD plane, the wavelength λ1 band image viewing uses the DMD operating in its +θ state with the λ1 PDA, with the option of individually time modulated the DMD pixels to its −θ state to reduce the PDA pixel exposure time to improve scene dynamic range. The wavelength λ3 band image viewing uses the DMD operating in its −θ state with the λ3 PDA, with the option again of the pixels being individually time modulated to its +θ state to reduce the PDA pixel exposure time to improve scene dynamic range. Thus, this configuration uses the DMD pixel control to select the desired wavelength band photo-detectors to be used for image acquisition. It should be noted that this system focuses on the usage of a small pixel count PDA, such as a FPA device, which are used for non-visible band applications. The mirror motion optics select the smaller pixel count scene from the full image on the DMD to transfer onto the smaller pixel count PDA chips. Hence, this system implements a compressive sampling of the input image scene present on the DMD.
It would be desirable that the entire large pixel count image could be detected with high dynamic range by the camera and inter-pixel crosstalk, pixel spill over due to saturation and flooding could be avoided or minimized to capture as much as possible of the true imaged scene when dealing with extreme imaging conditions involving bright lights within viewed scenes. In addition, it is desirable that the imager not only provides a high signal-to-noise ratio image but also provide a spatial imaging resolution that beats the classic Abbe diffraction limit to enable super-resolution imaging.
The present invention is concerned with overcoming at least some of the above mentioned problems with known imaging systems.